I figure Sentinals would be the large exspantion for MarvelScape.
Sentinals as the common hero, possibly put two or three to a large exspantion with some terrain(Possibly X-mansion themed)
Other large heros could include Ghost rider on his bike(I figure a motercycle to be roughly the size of a horse at least and could fit on a double hex base.)
Common squads,
Well we already have suggestions for Sheild and Hydra agents, as well as doombots, mole men, skrulls, brood, ect...
Maybe toss in some of those FoH thugs that would sometimes harras the X-Men, sure they would be about as effective as any angry mob would be against the likes of the hulk and so they would be low cost, maybe something like: 6 move, 1 attack, 1defence, 5 range and maybe give them scatter?